1. Field of the Invention
This invention pertains to microcomputer controlled telephone instruments and more particularly to a technique for reducing the number of ports utilized in connection with keypad detection and the detection of information on wired-in strap switches employed for inputting information to the microprocessor used in such a telephone instrument.
2. Background Art
A number of microprocessors are available commercially which have found appropriate use in the control of telephone instruments. One of these is Model No. COP444C made by National Semiconductor Corporation. This CMOS microcontroller includes an 8 pin input/output port, a 4 pin input/output port, a 4 pin output only port, a 4 pin input only port and a single serial input/output port. This arrangement distributed over 28 pins is representative of many microprocessors that would find useful application in telephone instruments. Another CMOS microcomputer, the NEC UPD7503 has 5 ports plus a serial input/output port and 2 interrupt pins in a 64 pin format. This unit includes an additional 28 pins for use as outputs to a liquid crystal display.
In a typical microprocessor controlled telephone instrument not including a liquid crystal display for example, an output port having 4 pins for peripheral device state control is required as are two 4 pin ports for an associated keypad plus strap switches, a single 8 pin output port plus a 4 pin port for random access memory and a 4 pin port for output to a DTMF (Dual Tone Multi-frequency) integrated circuit. In addition, a serial input/output port may be utilized for accessing a liquid crystal display driver arrangement. From the above it will be obvious that without multiplexing a minimum of 30 pins is usually required for use on a microcomputer involved in a microprocessor controlled telephone instrument. Clearly the above described units are insufficient in their capabilities to meet the requirement of today's state of the art designs.
Typical of state of the art telephone design is that exhibited in the "Hangtel" telephone manufactured by American Telecommunications Corporation. In this unit, a microprocessor manufactured by National Semiconductor and identified as COP411L which is quite similar the COP444C previously described, is employed. The utilization of the microprocessor is similar to the scheme outlined in an application note entitled COP Note 3, "Design Considerations for a COP420C-Based Telephone Line Powered Repertory Dialer" which appears on page 9-118 of the COPS Microcontrollers Data Book published by National Semiconductor Corporation in 1982.
Accordingly, it is the object of the present invention to provide a technique for effectively being able to utilize state of the art single-chip microcomputers having fewer than the necessary ports and count of pins that would normally be required, without sacrificing any of the features felt to be required in a microprocessor controlled telephone of current design.